Despite significant research effort, discogenic back pain remains prevalent in American society accounting for large economic and productivity losses in individuals disabled. The origins of discogenic back pain are complex with many contributing factors including, genetics, occupation and aging. Disc degeneration is characterized by a continual remodeling process where remodeling resulting in alterations of the disc composition and changes to the disc's mechanical properties lead to instability, decreased height and potentially pain. Mechanical load history has the ability to alter the metabolism of the disc and may accelerate remodeling and ultimately degeneration. The rat tail model has emerged with established methods to isolate and control a single spinal motion segment. Previous studies have shown that short term dynamic loads have resulted in altered disc metabolism while chronic compressive static loads lead to compositional changes within the disc including decreased axial stifhess, angular compliance and disc height. To date, no studies have examined the link between metabolic changes in the disc as a result of single event loading and compositional and structural level changes of the rat tail intervertebral disc as a result of chronic applied dynamic loading for periods in excess of two weeks. The goal of this study was, first, to develop and test new methods to apply a dynamic chronic compression load to the rat tail intervertebral disc lasting months or longer and, second, to evaluate measurements of compositional level dependent variables (GAG degradation, mechanical properties and histology) after a two week pilot study where dynamic force was applied 1.5 hours daily for 14 days.
A new method was developed and validated to apply compressive loading to the rat tail intervertebral disc. The rat tail compressive loading system utilized a pneumatically driven device weighing 18 g, which was capable of delivering 12.6 N sinusoidal and square waveforms at frequencies up to 1.0 Hz. The system improved on previous methods in its modular construction, relative ease of fabrication, compatibility with existing tail model technology and overall cost effectiveness. The removable system eliminated the need for anesthesia and through a modular design allowed for the simultaneous loading of multiple animals. The system was then used to apply a 1.0 MPa 1.0 Hz sinusoidal load to 12 rats for 1.5 hours daily for 14 consecutive days. At days 0, 7 and 14 a mechanical witness test was performed to evaluate the axial mechanical properties of the motion segment and disc tissue was harvested on day 15 (24 hours after the final loading). The results indicate that 1.5 hours loading for 14 days may be a threshold level where compositional changes within the disc are beginning to manifest and represent a remodeling response. Mechanical data indicate an increase in phase angle, possibly the result of a change in the matrix structure and compositional changes of increased GAG in the annulus fibrosus. Histologically, no structural change was evident and may indicate that two weeks is not sufficient time to induce structural changes. It is necessary for future studies to examine these same variables under more extreme loading conditions in an effort to induce structural changes consistent with degeneration.